The use of alkali metal silicates as a binder for foundry cores and molds has been known for a long time. More recently, carbon dioxide has been used to cure the silicate in a process known as the "Carbon Dioxide Process". When the CO.sub.2 gas is passed through the wet sand-silicate mixture after it has been fabricated into a desired shape, the gas combines chemically with the silicate binder to form a silicic acid gel which cements the sand grains together and accomplishes the desired binding in a short period of time. A fuller description of the CO.sub.2 process can be found in K. E. L. Nicholas, The CO.sub.2 -Silicate Process in Foundries (1972).
A major disadvantage encountered in using silicate bonded sand compositions is poor collapsibility after the metal has been poured in and solidified in the mold. In contrast to organic binders, the silicate does not burn out at low temperatures, but sinters with the sand at high temperatures to form glasses. Such sintering begins to occur at temperatures of about 1475.degree. F. and increases rapidly above about 1560.degree. F. As a consequence, collapsibility and shake-out features are generally very poor, and frequently inadequate, in sand molds and cores bonded with the alkali metal silicates, and the castings may be difficult to remove.
A better collapsibility can be obtained by reducing the quantity of silicate binder but this practice is undesirable because if the amount of silicate solution is too low, the molds and cores are exceedingly friable and are frequently unacceptable for general foundry use. Collapsibility can also be improved somewhat by incorporating a minor amount of certain filler or burn-out materials such as asphalt emulsions, cellulose fibers, cereal binders and the like, but these materials are a nuisance to work with and difficult to handle in the sand-silicate formulations. Additionally, such additives frequently increase the friability of the cores.
Another type of foundry binder compositions are classified "no-bake binders". These binders are capable of forming completely cured cores or molds at room temperature without the use of gaseous catalysts. Optionally, they can be baked to accelerate the curing.
More recently, sodium silicate binder systems have been developed which use simple organic esters for curing the silicate binder. Typical organic esters include diacetin, triacetin, glycol propionate, and mixtures thereof. 100% triacetin is reported to be slow, while 100% diacetin is reported to be very fast. Set-up times as short as 8.5 minutes to as long as 10 hours have been reported. See, Modern Casting, September 1974, pages 43 et seq.
It has now been found that the combination of an alkali metal silicate and a hydroxyl-terminated polyester provides a no-bake foundry binder system which rapidly cures and develops sufficient tensile strength. Foundry cores made by this new system reasonably withstand degradation from humidity and have the desired collapsibility of shake-out from the poured casting.
Accordingly, it is the object of this invention to provide a silicate no-bake binder foundry system which can be used to fabricate excellent quality foundry cores and molds having high strength and excellent collapsibility or shake-out characteristics. These and other objects of the invention will become apparent to those skilled in the art from the following detailed description.